1. Field of the Invention
The present invention relates to a sheet workpiece bending machine such as a sheet-metal bending press.
2. Description of the Prior Art
The cold bending of metal sheets is currently carried out by bending presses, an example of which is illustrated schematically in perspective in FIG. 1.
These presses include a frame 10 constituted by one or more strong, C-shaped structures. The frame 10 carries two strong, parallel steel beams 12 and 14, usually arranged in a vertical plane. One of the beams, for example the upper one 14, is movable so that it can be moved towards and away from the other beam 12 while remaining constantly parallel thereto. The double arrow Z indicates the direction of movement of the upper beam 14 or, at any rate, of the relative movement of the two beams. This direction will be referred to below conventionally as the "working direction".
The two beams 12 and 14 constitute respective tool holders for a pair of cooperating tools in the form of a die 16 and a punch 18 which are usually V-shaped. The sheet metal interposed between the die 16 and the punch 18 is bent into the shape of these tools when they are pressed against each other.
In conventional presses, the movement of the movable beam 14 and the force with which it is pressed against the other beam 12, which is usually fixed, are achieved by a mechanism which applies the force at a single point, if the machine is small (for bending lengths usually less than 1 m), or at two points situated symmetrically at the ends of the movable beam 14 in the case of medium and large-sized bending presses. This mechanism may be of various types and its force is usually developed by hydraulic cylinders or hydraulic motors.
In the case of high-precision bending presses, where the distance between the die and the punch must be adjusted accurately to produce a bending angle within strict tolerances (for example with an angular error of a few minutes of a degree) it is necessary to use numerically-controlled drive motors. This well-known technique requires the continuous and automatic measurement of the distance between the die 16 and the punch 18. In this case hydraulic drives (cylinders or motors), which are usually preferred for the high power which they can develop with components of a very limited size, are not very suitable. In fact, not only are the hydraulic slave mechanisms not very precise, but their performance also varies considerably during the course of a working day as the hydraulic fluid gradually heats up. Moreover, the large quantity of heat developed is transmitted to the frame of the machine and can cause deformation which reduces the precision of the system even more.
For the reasons set out above, the current tendency is to prefer electric servo-motors which are very precise and which perform consistently. Moreover, because of their very high efficiency, such servo-motors generate much smaller quantities of heat than those generated by hydraulic cylinders and motor.
The disadvantage of electric servo-motors lies in the fact that they are much more bulky and expensive than hydraulic drives, for a given power developed, and require kinematic mechanisms, such as the beam 14 of FIG. 1, which are also more expensive, for transmitting the drive to the movable tool holder.